Achieving high current densities while maintaining high energy efficiency is one of the biggest challenges in improving photoelectrochemical devices. Higher current densities accelerate the production of hydrogen and other electrochemical fuels.
Now a compact, solar-powered, hydrogen-producing device has been developed that provides the fuel at record speed. In the journal Nature Energy, the researchers around Saurabh Tembhurne describe a concept that allows capturing concentrated solar radiation (up to 474 kW/m²) by thermal integration, mass transport optimization and better electronics between the photoabsorber and the electrocatalyst.
The research group of the Swiss Federal Institute of Technology in Lausanne (EPFL) calculated the maximum increase in theoretical efficiency. Then, they experimentally verified the calculated values using a photoabsorber and an iridium-ruthenium oxide-platinum based electrocatalyst. The electrocatalyst reached a current density greater than 0.88 A/cm². The calculated conversion efficiency of solar energy into hydrogen was more than 15%. The system was stable under various conditions for more than two hours. Next, the researchers want to scale their system.
The produced hydrogen can be used in fuel cells for power generation, which is why the developed system is suitable for energy storage. The hydrogen-powered generation of electricity emits only pure water. However, the clean and fast production of hydrogen is still a challenge. In the photoelectric method, materials similar to those of solar modules were used. The electrolytes were based on water in the new system, although ammonia would also be conceivable. Sunlight reaching these materials triggers a reaction in which water is split into oxygen and hydrogen. So far, however, all photoelectric methods could not be used on an industrial scale.
2 H2O → 2 H2 + O2; ∆G°’ = +237 kJ/mol (H2)
The newly developed system absorbed more than 400 times the amount of solar energy that normally shines on a given area. The researchers used high-power lamps to provide the necessary “solar energy”. Existing solar systems concentrate solar energy to a similar degree with the help of mirrors or lenses. The waste heat is used to accelerate the reaction.
The team predicts that the test equipment, with a footprint of approximately 5 cm, can produce an estimated 47 liters of hydrogen gas in six hours of sunshine. This is the highest rate per area for such solar powered electrochemical systems. At Frontis Energy we hope to be able to test and offer this system soon.
At Frontis Energy we have spent much thought on how to recycle CO2. While high value products such as polymers for medical applications are more profitable, customer demand for such products is too low to recycle CO2 in volumes required to decarbonize our atmosphere to pre-industrial levels. Biofuel, for example from field crops or algae has long been thought to be the solution. Unfortunately, they require too much arable land. On top of their land use, biochemical pathways are too complex to understand by the human brain. Therefore, we propose a different way to quickly reach the target of decarbonizing our planet. The procedure begins with a desired target fuel and suggests a microbial consortium to produce this fuel. In a second step, the consortium will be examined in a bio-electrical system (BES).
Today’s atmospheric CO2 imbalance is a consequence of fossil carbon combustion. This reality requires quick and pragmatic solutions if further CO2 accumulation is to be prevented. Direct air capture of CO2 is moving closer to economic feasibility, avoiding the use of arable land to grow fuel crops. Producing combustible fuel from CO2 is the most promising intermediate solution because such fuel integrates seamlessly into existing urban infrastructure. Biofuels have been explored intensively in recent years, in particular within the emerging field of synthetic biology. However tempting the application of genetically modified organisms (GMOs) appears, non-GMO technology is easier and faster to implement as the required microbial strains already exist. Avoiding GMOs, CO2 can be used in BES to produce C1 fuels like methane and precursors like formic acid or syngas, as well as C1+ compounds like acetate, 2-oxybutyrate, butyrate, ethanol, and butanol. At the same time, BES integrate well into urban infrastructure without the need for arable land. However, except for methane, none of these fuels are readily combustible in their pure form. While electromethane is a commercially available alternative to fossil natural gas, its volumetric energy density of 40-80 MJ/m3 is lower than that of gasoline with 35-45 GJ/m3. This, the necessary technical modifications, and the psychological barrier of tanking a gaseous fuel make methane hard to sell to automobilists. To produce liquid fuel, carbon chains need to be elongated with alcohols or better, hydrocarbons as final products. To this end, syngas (CO + H2) is theoretically a viable option in the Fischer-Tropsch process. In reality, syngas precursors are either fossil fuels (e.g. coal, natural gas, methanol) or biomass. While the former is obviously not CO2-neutral, the latter competes for arable land. The direct conversion of CO2 and electrolytic H2 to C1+ fuels, in turn, is catalyzed out by electroactive microbes in the dark (see title figure), avoiding food crop competition for sun-lit land. Unfortunately, little research has been undertaken beyond proof of concept of few electroactive strains. In stark contrast, a plethora of metabolicstudies in non-BES is available. These studies often propose the use of GMOs or complex organic substrates as precursors. We propose to systematically identify metabolic strategies for liquid bio-electrically engineered fuel (BEEF) production. The fastest approach should start by screening metabolic databases using established methods of metabolic modeling, followed by high throughput hypothesis testing in BES. Since H2 is the intermediate in bio-electrosynthesis, the most efficient strategy is to focus on CO2 and H2 as direct precursors with as few intermediate steps as possible. Scalability and energy efficiency, economic feasibility that is, are pivotal elements.
Yeasts are among the microorganisms with the greatest potential for liquid biofuel production. Baker’s yeast, (Saccharomyces cerevisiae) is the most prominent example. While known for ethanol fermentation, yeasts also produce fusel oils such as butane, phenyl, and amyl derivate aldehydes and alcohols. Unlike ethanol, which is formed via sugar fermentation, fusel oil is synthesized in branched-off amino acid pathways followed by aldehyde reduction. Many enzymes involved in the reduction of aldehydes have been identified, with alcohol dehydrogenases being the most commonly observed. The corresponding reduction reactions require reduced NADH but it is not known whether H2 produced on cathodes of BES can be involved.
Clostridia, for example Clostridium acetobutylicum and C. carboxidivorans, can produce alcohols like butanol, isopropanol, hexanol, and ketones like acetone from complex substrates (starch, whey, cellulose, etc. ) or from syngas. Clostridialmetabolism has been clarified some time ago and is different from yeast. It does not necessarily require complex precursors for NAD+ reduction and it was shown that H2, CO, and cathodes can donate electrons for alcohol production. CO2 and H2 were used in a GMO clostridium to produce high titers of isobutanol. Typical representatives for acetate production from CO2 and H2 are C. ljungdahlii, C. aceticum, and Butyribacterium methylotrophicum. Sporomusa sphaeroides produces acetate in BES. Clostridia also dominated mixed culture BESs converting CO2 to butyrate. They are therefore prime targets for low cost biofuel production. Alcohols in clostridia are produced from acetyl-CoA. This reaction is reversible, allowing acetate to serve as substrate for biofuel production with extracellular energy supply. Then, energy conservation, ATP synthesis that is, can be achieved from ethanol electron bifurcation or H2 oxidation via respiration. While possible in anaerobic clostridia, it is hitherto unknown whether electron bifurcation or respiration are linked to alcohols or ketone synthesis.
Phototrophs like Botryococcus produce C1+ biofuels as well. They synthesize a number of different hydrocarbons including high value alkanes and alkenes as well as terpenes. However, high titers were achieved by only means of genetic engineering, which is economically not feasible in many countries due to regulatory constrains. Moreover, aldehyde dehydration/deformylation to alkanes or alkenes requires molecular oxygen to be present. Also the olefin pathway of Synechococcus depends on molecular oxygen with the cytochrome P450 involved in fatty acid decarboxylation. The presence of molecular oxygen affects BES performance due to immediate product degradation and unwanted cathodic oxygen reduction. In contrast, our own preliminary experiments (see title photo) and a corrosion experiment show that algae can live in the dark using electrons from a cathode. While the enzymes involved in the production of some algal biofuels are known (such as olefin and aldehyde deformylation), it is not known whether these pathways are connected to H2 utilization (perhaps via ferredoxins). Such a connection would be a promising indicator for the possibility of growing hydrocarbon producing cyanobacteria on cathodes of BES and should be examined in future research.
At Frontis Energy we believe that a number of other microorganisms show potential for BEEF production and these deserve further investigation. To avoid GMOs, BES compatible co-cultures must be identified via in silico metabolic reconstruction from existing databases. Possible inter-species intermediates are unknown but are prerequisite for successful BES operation. Finally, a techno-economical assessment of BEEF production, with and without carbon taxes, and compared with chemical methods, will direct future research.
Recently, we reported on plans by Australian entrepreneurs and their government to use ammonia (NH3) to store excess wind energy. We proposed converting ammonia and CO2 from wastewater into methane gas (CH4), because it is more stable and easier to transport. The procedure follows the chemical equation:
8 NH3 + 3 CO2 → 4 N2 + 3 CH4 + 6 H2O
Now we have published a scientific article in the online magazine Frontiers in Energy Research where we show that the process is thermodynamically possible and does indeed occur. Methanogenic microbes in anaerobic digester sludge remove the hydrogen (H2) formed by electrolysis from the reaction equilibrium. As a result, the redox potentials of the oxidative (N2/NH3) and the reductive (CO2/CH4) half-reactions come so close that the process becomes spontaneous. It requires a catalyst in the form of wastewater microbes.
To prove our idea, we first searched for the right microbes that could carry out ammonia oxidation. For our experiments in microbial electrolysis cells we used microorganisms from sediments of the Atlantic Ocean off Namibia as starter cultures. Marine sediments are particularly suitable because they are relatively rich in ammonia, free from oxygen (O2) and contain less organic carbon than other ammonia-rich environments. Excluding oxygen is important because it used by ammonia-oxidizing microbes in a process called nitrification:
2 NH3+ + 3 O2 → 2 NO2− + 2 H+ + 2 H2O
Nitrification would have caused an electrochemical short circuit, as the electrons are transferred from the ammonia directly to the oxygen. This would have bypassed the anode (the positive electron accepting electrode) and stored the energy of the ammonia in the water − where it is useless. This is because, anodic water oxidation consumes much more energy than the oxidation of ammonia. In addition, precious metals are often necessary for water oxidation. Without producing oxygen at the anode, we were able to show that the oxidation of ammonium (the dissolved form of ammonia) is coupled to the production of hydrogen.
It was important that the electrochemical potential at the anode was more negative than the +820 mV required for water oxidation. For this purpose, we used a potentiostat that kept the electrochemical potential constant between +550 mV and +150 mV. At all these potentials, N2 was produced at the anode and H2 at the cathode. Since the only source of electrons in the anode compartment was ammonium, the electrons for hydrogen production could come only from the ammonium oxidation. In addition, ammonium was also the only nitrogen source for the production of N2. As a result, the processes would be coupled.
The reaction produces CO2 and methane at a ratio of 1:1. Unfortunately, the CO2 in the biogas makes it almost worthless. As a result, biogas is often flared off, especially in places where natural gas is cheap. The removal of CO2 would greatly enhance the product and can be achieved using CO2 scrubbers. Even more reduced carbon sources can shift the ratio of CO2 to CH4. Nevertheless, CO2 would remain in biogas. Adding hydrogen to anaerobic digesters solves this problem technically. The process is called biogas upgrading. Hydrogen could be produced by electrolysis:
2 H2O → 2 H2 + O2; ∆G°’ = +237 kJ/mol (H2)
Electrolysis of water, however, is expensive and requires higher energy input. The reason is that the electrolysis of water takes place at a relatively high voltage of 1.23 V. One way to get around this is to replace the water by ammonium:
2 NH4+ → N2 + 2 H+ + 3 H2; ∆G°’ = +40 kJ/mol (H2)
With ammonium, the reaction takes place at only 136 mV, which saves the respective amount of energy. Thus, and with suitable catalysts, ammonium could serve as a reducing agent for hydrogen production. Microorganisms in the wastewater could be such catalysts. Moreover, without oxygen, methanogens become active in the wastewater and consume the produced hydrogen:
The low energy gain is due to the small potential difference of ΔEh = +33 mV of CO2 reduction compared to the ammonium oxidation (see Pourbaix diagram above). The energy captured is barely sufficient for ADP phosphorylation (ΔG°’ = +31 kJ/mol). In addition, the nitrogen bond energy is innately high, which requires strong oxidants such as O2 (nitrification) or nitrite (anammox) to break them.
Instead of strong oxidizing agents, an anode may provide the activation energy for the ammonium oxidation, for example when poised at +500 mV. However, such positive redox potentials do not occur naturally in anaerobic environments. Therefore, we tested whether the ammonium oxidation can be coupled to the hydrogenotrophic methanogenesis by offering a positive electrode potential without O2. Indeed, we demonstrated this in our article and have filed a patent application. With our method one could, for example, profitably remove ammonia from industrial wastewater. It is also suitable for energy storage when e.g. Ammonia synthesized using excess wind energy.